The specific correction of the mechanical loading circumstances at the knee in a case of one-sided degenerated illness is a known and often applied method to regain freedom from pain and to enhance mobility without providing a prosthesis. In an orthopedic surgical operation, the position of the axis of the lower extremity is altered so that a relief of stress on the affected joint segment is achieved. The most commonly applied variant of this operation is the lateral set-on valgusating tibia-head-osteotomy when there is an existing impairment of the medial joint portion. In this procedure, an alteration of the axis of the leg toward knock knees is produced through removal of a wedge-shaped bone chip producing release of the ill medial joint compartment. Conditions for the clinical success of the adjustment osteotomy are, apart from the correct indication position, the exact determination of the correcting angle, the removal of a wedge-shaped bone chip corresponding to the correction angle, the quality of the areas of osteotomy and the stable micro-motion-free osteosynthesis by means of a suitable osteosynthesis implant.
In the case of a medial gonarthrosis, the mechanical axis that forms a straight line through the joint centers of the hip, knee and ankle joints in a healthy knee is displaced through the rearrangement by 20 to 30 mm toward the lateral joint part. The correction angle is the sum of the existing false position and an over-correction of the mechanical axis. Because of that over-correction, the load on the ill joint half is reduced and the opposite, healthy half is loaded a little more strongly. The bone wedge which is to be removed can either be removed horizontally, i.e., perpendicular to the axis of the diaphysis, or at an angle to it. When using the mentioned instruments, we choose an osteotomy course which is inclined rising toward the ill side to determine the rotating point of the osteotomy in the compact subchondral bone a few millimeters from the lateral limiting corticalis. The bone bridge so attained produces a tension boom and allows the simple osteosynthesis of the osteotomy by means of a reciprocal attached implant. For the production of this bone bridge, which is important for stability, the final choice of the general osteotomy angle relative to the joint axis is important since the crossing point of the wedge-shaped osteotomy in the compact bone volume directly below the joint provides good conditions for undisturbed healing without secondary correction loss.
For planning the operation, one uses an x-ray photograph of the full length of the extremity, centered on the joint, with the plane of the photograph corresponding with the frontal plane as exactly as possible to avoid projection errors. When taking measurements from the x-ray photograph, the factor of enlargement must be considered. Because x-ray photographs are affected by a nonlinear enlargement factor going out from the center of the ray, and the plane of the photography lies as a rule not exactly in the axis of the body because of position inaccuracies, this planning is always affected with faults. With patients of advanced age, the photography technique is often especially difficult. With given contraction of flexion, minor differences of rotation from the frontal plane have such consequences that the accuracy which is necessary to reach an optimal result is not given in planning. For conversion of the correction angle from the planning to determination of the guidance of the sawing tool angle, it is customary to use instruments which are attached to the skeleton and are provided with arrangements for guidance of a mechanically driven saw blade.
The authors of EP application 0 231 885 suggest the attachment of an instrument shaped as a circular arc and scaled in angular degrees by means of two pins which are positioned above and below the osteotomy in the tibia. The pins are positioned relative to each other along lines representing the wedge angle of the osteotomy. These pins carry a plate which forms the guide for the saw when positioned against the bone. With this, the angle and the plane of the wedge-shaped converging osteotomy planes are determined. Not determined is the location of the coincidence of the saw cuts at the peak of the bone wedge which is to be removed. This should, in the ideal case, be set some millimeters before the opposite cortical shell in the compact joint close spongiosa of the tibia so that, on one side, the osteotomy areas are apt to fit over the whole cross-section of the bone and on the other side, a minimal bone volume connected with shortening of the leg is removed, as little as is possible. Preservation of a thin deformable bone bridge is desired and ensures certain allocation of the osteotomy segments and increases the stability of the subsequent osteosynthesis in the sense of a tension boom. With the instrument suggested in EP 0 231 885, this crossing location is to be determined from the x-ray photograph and the instrument must be installed during the operation at the tibia with distance and orientation according to the planning. In this realization, the location of the cut edge is the central point of the angle-determining circular arc segment of the instrument.
In French patent application 2,679,126, a similar arrangement is described which is likewise attached to the skeleton by pins and allows osteotomies which permit the removal of a bone wedge sequently with a saw guide including a guide rail in the shape of a circular arc segment which exactly guides the saw, the saw being a reciprocating bone saw pivotable in angular degrees. In this device, the crossing location of the saw cuts must be planned on an x-ray photograph and transferred to the situation during the operation.
As described above, planning using an x-ray photograph contains some inevitable inaccuracies. In addition, it is to be expected that, when positioning one of the above described instruments on the patient, further inaccuracies will appear as to the location of the osteotomy wedge, particularly as the result of the fact that the positioning is not directly examinable in advance.